Sliding stem control valves are commonly used to control fluid flow within process control loops. As known to those of ordinary skill in the art, a sliding stem control valve modulates fluid flow through a valve body by using an actuator, coupled through a bonnet assembly, to move a control element or valve plug in a reciprocal motion relative to a valve seat. The seat ring provides an annular surface within the valve body against which the valve plug engages to close off the valve when desired. Two important features of a sliding stem control valve are that it must control or modulate the fluid in a predetermined manner, including providing tight shut off when closed, and it must be capable of withstanding the pressure and temperature influences of the process.
Additionally, conventional sliding stem control valves provide some form of guiding to align the valve plug and the valve seat. One type of guiding known to those skilled in the art is provided by a bushing or guide surface positioned within the bonnet assembly. A reduced diameter of the valve plug bears upon a bushing mounted inside a journal in the bonnet assembly. This type of guiding is known as post guiding. As known to those of ordinary skill in the art, standard manufacturing tolerances and conventional assembly procedures can produce a misalignment of the centerline of the valve seat relative to the valve plug guiding. This misalignment or offset in concentricity does not allow the valve plug to properly seat within the valve seat, thus providing a substantial leak path between the valve plug and seat during shut off or valve closure. In certain applications, the concentricity problem will reduce shut-off performance and may create high velocity flows that erode the valve seat further degrading valve performance.
Another typical type of guiding, also known to those skilled in the art as post guiding, is where the valve plug is guided by the valve trim (i.e. the internal valve components exposed to the process). Generally, a seat ring retainer provides an internal guiding surface within the fluid flow path on which the valve plug moves throughout its full stroke. In such a valve construction, a separate seat ring retainer and seat ring are typically clamped in place between the valve body and bonnet. A gasket, such as a spiral-wound gasket, is generally provided between the bonnet and the seat ring retainer in conventional clamped-trim designs. As known to those skilled in the art, the clamped trim provides an engineered, pre-loaded force to compress the spiral-wound gasket. Under compression, the spiral-wound gasket provides an opposing spring force that creates a seal between the seat ring retainer and the bonnet and the seat ring and the body. These designs are susceptible to seal failures as described in greater detail below. Those skilled in the art recognize three specific problems associated with conventional gaskets or seals.
First, sliding stem control valves are frequently used in process applications encountering temperatures in excess of 300 degrees Fahrenheit and pressure drops exceeding 150 psi. These extreme operating conditions create valve assembly leakage problems due to differences in the thermal expansion of the materials of construction. By using different materials of construction, valve manufacturers can increase performance and/or decrease cost of the valve assembly. For example, general process applications may employ a control valve having a steel body and stainless steel seat ring retainer. As is known, the steel valve body has dissimilar thermal expansion characteristics of the stainless steel seat ring retainer. Thus, an increase in operating temperature may subsequently increase the pre-load force such that the spiral-wound gasket is overstressed or yields resulting in permanent deformation. When the temperatures return to ambient conditions, the gasket cannot return to its pre-stressed condition and a leak path is created. Alternatively, if a valve body is constructed of a material that expands more than the seat ring retainer material, an increase in operating temperature may cause a corresponding decrease in the pre-load force on the spiral wound gasket causing a leak between the seat ring and the valve body.
Second, conventional sliding stem valves that utilize spiral-wound gaskets are subject to pressure limitations. The compliant nature of the spiral-wound gasket creates a robust seal for numerous applications, but as those skilled in the art recognize, these gasket are limited in the pressure drop or differential pressure that can be accommodated without losing seal integrity. For example, if the spiral-wound gasket in an unbalanced control valve is able to maintain adequate trim clamping force up to a pressure limit of 300 psi, any differential pressure exceeding that pressure limit may temporarily deform the gasket and subsequently creating a leak path.
Third, leaks are addressed in conventional clamped-design sliding stem control valve assemblies by applying tremendous loading in the bonnet bolting. As known to those of ordinary skill in the art, increasing the bonnet bolting loading increases control valve cost by requiring heavier structures within the valve body and bonnet to withstand the increased loading as well as requiring larger bonnet bolting to apply the loads.
It can therefore be seen that a need exists for an improved seat ring assembly for a sliding stem control valve which is manufactured from fewer pieces, and which can be assembled and maintained in position with less hardware and structure than is currently demanded.